The goal of this proposal is to study the unique ultrastructure of the Legionnaires' disease agent (LA), and to study antigens associated with surface of the organism. These antigens will be isolated, purified, and characterized by physical-chemical methods and monitored for toxicity. Rapid-diagnostic tests will be developed to detect either antigens or antibodies in specimens. Characterization of functional antibodies by bactericidal and opsonophagocytic assays will be done to define which antigen (s) is (are) important in immunity to LA. Data obtained from the serologic testing will be used to serotype the LA and antigens that absorb functional antibody activity will be considered for future investigation as a vaccine.